1. Field of the Invention
The present invention relates to a laser tracking interferometer.
2. Description of Related Art
A fixed-datum type laser tracking interferometer is known (refer to U.S. Pat. No. 6,147,748, Japanese Patent No. 2603429, and Japanese Patent No. 4776454, for example), in which a first retroreflector as a measured body is attached at a front end of a Z axis or the like of a three-dimensional measuring apparatus and a laser beam is emitted toward the first retroreflector. Interference of the laser beam reflected by the first retroreflector in a return direction (direction opposite to emission) is used to measure a change in a distance from the center of a reference sphere, which is a reference point of measurement, to the first retroreflector. Tracking is also performed based on a change in a position of an optical axis of the laser beam.
Furthermore, a laser interferometer is known which measures a change in a distance between two points opposite to each other with the interferometer therebetween (refer to Japanese Patent Laid-Open Publication No. H7-190714, for example).
FIG. 1 is a schematic configuration of a main portion of a laser tracking interferometer disclosed in Japanese Patent No. 4776454. In a device illustrated in FIG. 1, a first retroreflector 105 as a measured body is attached at a front end of a Z axis or the like of a three-dimensional measuring apparatus. The device tracks the first retroreflector 105 moving in space and measures in a highly accurate manner an amount of change ΔL in a distance L from a center point C of a reference sphere 101 to the first retroreflector 105, the reference sphere 101 having excellent sphericity and being fixed in space.
The reference sphere 101 is produced such that a radius thereof is identical around an entire surface in a highly accurate manner. Thus, the amount of change ΔL in the distance L can be obtained from an amount of change ΔL2 and an amount of change ΔL1 as shown below, the amount of change ΔL2 being measured by a displacement gauge 103 fixated onto a carriage 102 rotating around the point C, the amount of change ΔL1 being measured by a laser interferometer 104 similarly fixated onto the carriage 102.ΔL=ΔL1+ΔL2 
The amount of change ΔL2 represents an amount of change in a distance L2 from the surface of the reference sphere 101 to a reference point P2 of displacement measurement of the displacement gauge 103. The amount of change ΔL1 represents an amount of change in a distance L1 from a reference point P1 of displacement measurement of the laser interferometer 104 to the first retroreflector 105.
A situation is assumed herein where, for example, a general Michelson interferometer is used as the laser interferometer 104, which is fixated to the carriage 102 at a connection point P on the carriage 102.
In a state where the first retroreflector 105 stands still without moving in space, when a housing of the laser interferometer 104 undergoes thermal expansion due to a change in surrounding temperature or the like, an amount of change ΔL4 is generated in a distance L4 from the connection point P to the reference point P1, and thus the distance L4 from the connection point P to the reference point P1 increases by the amount of change ΔL4. As a result, even though the first retroreflector 105 stands still and the distance L1 from the reference point P1 to the first retroreflector 105 is not supposed to change, the distance L1 is measured shorter by the amount of change ΔL4 since a position of the reference point P1 of the laser interferometer 104 is pushed out and changed. Thus, when the housing of the laser interferometer 104 undergoes thermal expansion due to a change in surrounding temperature or the like, a problem arises where an error occurs in a measurement value of the amount of change ΔL.
Similarly, when the carriage 102 undergoes thermal expansion due to a change in surrounding temperature or the like, an amount of change ΔL3 is generated in a distance L3 from the connection point P to the reference point P2 of displacement measurement of the displacement gauge 103, and thus the distance L3 from the connection point P to the reference point P2 increases by the amount of change ΔL3. As a result, even though the first retroreflector 105 stands still and the distance L2 from the reference point P2 to the reference sphere 101 is not supposed to change, the distance L2 is measured shorter by the amount of change ΔL3 since a position of the reference point P2 of the displacement gauge 103 is pushed out and changed. Thus, when the carriage 102 undergoes thermal expansion due to a change in surrounding temperature or the like, a problem arises where an error occurs in a measurement value of the amount of change ΔL.